Re: performance of virtual functions compared to virtio

[Avi Kivity <avi@redhat.com>]

On 04/25/2011 08:49 PM, David Ahern wrote:
> >
> >  There are several copies.
> >
> >  qemu's virtio-net implementation incurs a copy on tx and on rx when
> >  calling the kernel; in addition there is also an internal copy:
> >
> >          /* copy in packet.  ugh */
> >          len = iov_from_buf(sg, elem.in_num,
> >                             buf + offset, size - offset);
> >
> >  In principle vhost-net can avoid the tx copy, but I think now we have 1
> >  copy on rx and tx each.
>
> So there is a copy internal to qemu, then from qemu to the host tap
> device and then tap device to a physical NIC if the packet is leaving
> the host?

There is no internal copy on tx, just rx.

So:

   virtio-net: 1 internal rx, 1 kernel/user rx, 1 kernel/user tx
   vhost-net: 1 internal rx, 1 internal tx

> Is that what the zero-copy patch set is attempting - bypassing the
> transmit copy to the macvtap device?

Yes.

> >
> >  If a host interface is dedicated to backing a vhost-net interface (say
> >  if you have an SR/IOV card) then you can in principle avoid the rx copy
> >  as well.
> >
> >  An alternative to avoiding the copies is to use a dma engine, like I
> >  mentioned.
> >
>
> How does the DMA engine differ from the zero-copy patch set?

The DMA engine does not avoid the copy, it merely uses a device other 
than the cpu to perform it.  It offloads the cpu but still loads the 
interconnect.  True zero-copy avoids both the cpu load and the 
interconnect load.

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